1. Field of the Invention
The present invention relates to microchannel plate image intensifiers (MCPIs), and more particularly, to a method for improving the gain uniformity of MCPIs.
2. Description of Related Art
A microchannel plate image intensifier (MCPI) comprises a photocathode (which converts input photons to electrons), a microchannel plate (which amplifies the number of electrons), and a phosphor screen (which converts the electron current back into photons). Gain uniformity in microchannel plate image intensifiers is important when measuring small changes in intensity. To improve a system's ability to detect these small intensity changes involves correcting for spatial non-uniformity in the imaging system, a process which reduces the system's dynamic range. Increasing the digitizing resolution (more digitizing bits) to compensate for a dynamic range loss is difficult due to the high data rate required to transmit the data in some applications. The alternative is to improve uniformity of the imaging system so that the dynamic range loss will be minimized during non-uniformity correction. Microchannel plate image intensifier (MCPI) non-uniformity dominates system non-uniformity. Tube manufacturers have had difficulty in producing MCPIs with non-uniformities of +10% or better.
Scrubbing is a term used to describe the process for removing surface gas molecules residing in the channels of a microchannel plate (MCP) or trapped in the phosphor screen area. Gas molecules inside the channels can be struck and ionized by electrons traveling from the input to the output of the MCP. These ions will be accelerated towards the input by the electric field. When they encounter the MCP wall, secondary electrons can be generated, causing a net increase in secondary electrons, which, in turn, can dislodge and ionize more gas molecules (similar to positive feedback). The result is an unstable gain characteristic of the MCP, where gain increases with input electron current. Initial MCP gain can be in the order of 10,000 and is reduced to about 1,000 by scrubbing, where gain becomes stable, not increasing as input current is increased. During vacuum processing of a MCPI, and before the cathode is formed and sealed to the tube body, the MCP-Screen assembly is "scrubbed" with electrons from either an electron gun or a temporary "scrubbing" photocathode. Voltages are applied to the MCP and screen regions. The electrons are accelerated to the MCP input where they strike, causing release of secondaries, as in normal tube operation. The electrons cascade through the MCP and strike the screen. On their journey, they encounter gas molecules, striking and ionizing some of them as they proceed. The ionized molecules are accelerated toward the MCP input by the electric field and are pumped out of the chamber by the vacuum system. One scrubbing process used by industry involves incrementally increasing MCP voltage and changing the input electron current over a period of approximately four hours, ending with an output current density of approximately 0.4 mA/cm.sup.2.
Besides causing unstable gain characteristics in the MCP, in a sealed tube, the gas molecules leaving the MCP input will be accelerated to the cathode where they collide with cathode material and damage the cathode, reducing its sensitivity. This effect damages all cathodes, but is particularly devastating to GaAs (Gen III) cathodes, where, in addition to electron scrubbing, a film is applied to the MCP input surface to block the ions as they leave. It would be beneficial if this mechanism for damaging cathode sensitivity could be used to improve the uniformity of sealed tubes. The present invention provides this benefit.